But, increasingly, the age of fathers is being scrutinised too. Simon Cowell's late entry to fatherhood at the age of 54 has coincided with the publication of research linking older fathers (45 years and older) to having children with an increased risk of autism, attention deficit disorder, bipolar disorder, substance abuse and low academic achievement. So is it safer for fathers to have babies earlier?

The solution

It has been known for a while that as men get older, their sperm are likely to develop mutations. This is because the sperm cells constantly divide, and the more they do so, the more likely they are to make errors. This is usually a single mistake that causes a recognised syndrome. One example is Crouzon syndrome, a rare condition in which the skull bones fuse too early and restrict growth. These errors become more likely over the age of 40. Evidence is gradually emerging that sperm mutations may also increase other risks, such as miscarriages, birth defects and mental health problems in later life.

This latest study followed up the medical and educational records of more than 2.6 million babies born between 1973 and 2001 and looked at the different rates of mental health problems between siblings from the same father. It found increased rates of autism and mental heath problems in children of older fathers. For example, 1% of those born to fathers under the age of 45 had bipolar disorder, but this rose to about 14% in their siblings if they were born when the fathers were over 45. But, compelling though the figures are, the study has limitations. For instance, it can't say if the differences were caused by how differently children were treated in the same family.

So studies so far should not frighten you into having children earlier. Most people can't know when they'll meet someone with whom they want to have children. There are also benefits of later parenthood, such as more financial security and emotional maturity. But if you do put off having children, there is one big risk – being mistaken for a grandparent in the playground.